Your unborn baby's lungs have been "practicing" breathing for several weeks. Since week 17, he's been taking amniotic fluid into his lungs and expelling it, in breathlike motions. His lungs are now mature enough to take the next step toward taking in air. At week 24, your baby-to-be begins to produce surfactant, an important fluid necessary for your baby to breathe outside the womb.

To understand the importance of surfactant, you must also have some idea about how the lungs develop and function. Lungs extend out like trees -- both sides have a main "trunk" that then branches out into smaller parts. All of these small parts give the lungs greater surface area from which to draw in oxygen, absorb it, and force out carbon dioxide. The smallest parts of the lung, on the ends of the extending "branches," act like little balloons filling with air, then emptying. These small structures within the lungs produce surfactant, a complex fluid that helps air enter and exit the lungs more easily. Surfactant acts as a lubricator. In other words, it decreases the surface tension of the lungs. To explain how this works, think of the lungs as a sieve and air as sand. If the sieve were lined with sandpaper, the sand would move slowly and might even stall on the coarse surface. But if the sieve were made of smooth, oiled plastic, the sand would pass through quickly.

Along with development in your baby-to-be's lungs, he's continuing to add fat tissue. You should be feeling your baby's movements more and more with his increased weight.

Terms to Know

Surfactant: A fluid produced by the alveoli and bronchioles of the lungs to aid in oxygen passage into and out of the body.